Axial-piston drive system with a continuously adjustable piston stroke

ABSTRACT

The invention relates to an axial piston drive with a continuously adjustable piston stroke, which comprises a drive shaft ( 10, 12 ) on which a swash plate ( 16 ) is supported in a crank chamber ( 14 ) in such a way as to be tiltable and displaceable in the axial direction, as well as a controller ( 18, 20 ) by means of which an tilt angle and an axial position of the swash plate ( 16 ) can be adjusted, and at least one piston ( 26, 28 ) connected to the swash plate ( 16 ) so that it can be actuated to move within a cylinder ( 22, 24 ).

CROSS REFERENCE TO RELATED APPLICATION

The present application is the U.S. national stage application ofInternational Application PCT/EP00/08084, filed Aug. 18, 2000, whichinternational application was published on Feb. 22, 2001 asInternational Publication WO 01/12989 A1 in the German language. TheInternational Application claims priority of German Patent Application199 39 131.9, filed Aug. 18, 1999.

STATE OF THE ART

The invention relates to an axial piston drive with a continuouslyadjustable piston stroke.

The use of axial piston drive with a continuously adjustable pistonstroke is known in particular for motor-vehicle air conditioners, wherethey serve as coolant condensers.

The main components of an air conditioner for a motor vehicle are acoolant condenser, a first heat exchanger, the so-called evaporator, asecond heat exchanger, an expansion organ and conduits that connect thecomponents to one another. The role of the coolant condenser is to sucka coolant out of the evaporator, in which the coolant evaporates underheat absorption, and to condense it at a higher pressure level.Subsequently, in the second heat exchanger, the coolant can release theheat at a higher temperature level, and in the expansion organ it isreturned to a pressure level corresponding to that of the evaporator.

The output of the coolant condenser can be continuously adjusted by wayof the speed of a drive motor and in an especially energeticallyfavourable manner, in the case of axial piston drives, by way of thepiston stroke. Known axial piston drives or axial piston condensers forvehicle air conditioners comprise a drive shaft operated by way of apulley. Within a crank chamber a swash plate is supported on the driveshaft so that it is unrotatably fixed and can be tilted by way of ajoint. The swash plate drives at least one piston that can move within acylinder. In order to absorb tractive and pressure loads, each suchpiston is connected to the swash plate by way of two hinge yokes, one atthe bearing surface of the swash plate that faces the piston and theother at the surface that faces away. With their flat surfacescontacting the bearing surfaces of the swash plate, the hinge yokes runat full circumferential velocity with a superimposed radial movement,which results in an elliptical path. The hinge yokes are seated withtheir rounded surfaces in sphere shaped formed bearings of the pistons,within which there is comparatively little relative movement duringoperation.

Furthermore, the connection between the swash plate and the piston canbe formed by way of a wobble plate rather than hinge yokes as describedabove. The wobble plate is secured against rotation with respect to thedrive shaft by either a housing or piston rods. A bearing between theswash plate and the wobble plate absorbs the entire relative movement.The wobble plate performs only a wobbling movement as a result of therotation of the swash plate.

The piston stroke and hence the output of the axial piston drive unit isadjusted by altering the tilt angle of the swash plate. A large tiltangle results in a long piston stroke and high output, whereas with asmall tilt angle the piston stroke is shorter and the output lower. As arule, the tilt angle of the swash plate is limited to a minimal and amaximal value by two stops. Ordinarily one or two guide pins are neededto guide the tilting movement in a specified manner and to avoidjamming. The tilt limiters, i.e. the stops, can be integrated into theguide pins.

If, as a result of adjustment of the tilt angle from a maximal value toa smaller one, a top-dead-centre point of the piston within the cylinderis shifted in the direction of the swash plate, already compressed gascannot be completely expelled. The compression energy introduced intothe gas cannot be utilized for the cooling process. The result is a“damage space” between the piston and a valve plate on the cylinder,which causes a loss of energy. In order to avoid the damage space and tokeep the top-dead-centre point of the piston in a constant position, theswash plate is supported so that it can additionally be axiallydisplaced against a prestressed compression spring. The movement of theswash plate in the axial direction is usually limited by stops.

ADVANTAGES OF THE INVENTION

The axial piston drive in accordance with the invention comprises adrive shaft with a swash plate supported thereon within a crank chamberso as to be tiltable and displaceable in the axial direction. A tiltangle and an axial position of the swash plate can be adjusted by acontroller. The driving action of the swash plate is exerted byconnection to at least one piston that can move within a cylinder.

It is proposed that the controller comprise an adjustment unit separatefrom the piston. With such a separate adjustment unit it is possible toobtain a large range of control that is independent of the operatingpoints. A controlling torgue can be applied exclusively in the directionof the possible adjustment movement of the swash plate, which enablesjamming and increased wear and tear to be avoided.

Flow losses between the upper side of the piston and the crank chambercan also be avoided and the full output of the condenser can beexploited, for example for cooling an air conditioner. Moreover, theaxial piston drive can be operated with low pressure in the crankchamber. A leakage flow of coolant from the crank chamber and outwardthrough shaft seals is approximately proportional to the pressure in thecrank chamber. By keeping the pressure low, an elaborate sealing of thecrank chamber can be eliminated and the leakage current made smaller.This is advantageous in particular in the case of coolants with highabsolute pressures, for which in general high pressures in the crankchamber are needed to achieve control by way of a gas-pressuredifference at the piston. With a low pressure, furthermore, the coolantof an air conditioner is only slightly soluble in a lubricant of thecondenser, as a result of which a high viscosity can be maintained.

Another way in which a separate adjustment unit has a positive effect onviscosity is that heating of the lubricant by gas that has been warmedby the high-pressure side of the piston can be avoided. With a highviscosity, low friction between heavily loaded pairs of sliding elementson the swash plate and between the pistons and the cylinders can beachieved, which contributes to a long working life and a high degree ofreliability.

With an adjustment unit separate from the piston, no particular pressurein the crank chamber is needed for control, as a result of which coolantcan be conducted from an evaporator through the crank chamber into thecylinder. Therefore the crank chamber can be cooled, an additionalsuction chamber on the upper side of the piston can be avoided, andhence the whole structure occupies less space. Furthermore, it isusually possible to utilize a large volume of the crank chamber for theattenuation of gas pulsations.

The adjustment unit can be driven electrically, pneumatically orpreferably hydraulically. With hydraulic fluid an advantageous dampingof oscillation can be achieved and a particularly vibration-insensitiveaxial piston drive created. The hydraulic adjustment unit can besupplied with compressed oil by a hydraulic unit that is independent ofthe medium being propelled by the piston; for example, a hydraulic unitthat is already present in a motor vehicle can advantageously be usedfor this purpose. Additional components can then be eliminated and alarge range of control, independent of the operating points of the axialpiston drive, can be attained. Furthermore, no build-up of pressure isneeded for control when the axial piston drive is started up, forinstance through a minimal tilt angle of 2°. A load-free starting of theaxial piston drive is made possible, and it becomes easier to startdevices such as an internal combustion engine that powers the axialpiston drive.

With an oil trap connected downstream of the condenser, good heattransfer into the heat exchanger can be ensured and a high efficiency ofan air conditioner achieved. Furthermore, the oil trap can be put toparticularly good use if it supplies the hydraulic adjustment unit withcompressed oil. Pressure is applied to the oil from the oil trap to anextent that depends on the operating point. If a large controllingtorgue is required, the pressure in the oil trap is high; if only asmall controlling torgue is needed, the pressure there is low.

In one embodiment it is proposed to connect the hydraulic adjustmentunit to the crank chamber by way of a drain, which is a particularlyuseful arrangement in that the oil trap and the adjustment unit can beused to transport the lubricant back into the crank chamber. In thisprocess, a influx from the oil trap to the adjustment unit and/or thedrain from the adjustment unit to the crank chamber can be madecontrollable. If only the drain or the influx is designed to becontrollable, whichever of these is not controlled can incorporate aninexpensive throttling site.

In the case in which only the drain or the influx can be controlled, itmay happen that more lubricant is separated out in the oil separatorthan is needed for the adjustment unit or for the control. To ensurethat the amount of lubricant in the crank chamber is always appropriate,in one embodiment it is proposed that there be disposed in the oilseparator and/or in the crank chamber at least part of an oil-levelcontroller which, when an oil level in the oil separator is exceededand/or the oil in the crank chamber falls below a certain level,connects the oil separator to the crank chamber by way of a channel. Itis further possible for the oil separator to be permanently connected tothe crank chamber by a channel and a throttling site, or the oilseparator and the amount of oil coordinate with respect to one anotherin such a way that the oil separator overflows before an oil orlubricant deficiency develops in the crank chamber. The overflowing oilcan subsequently be sent into the crank chamber, for example togetherwith a coolant for an air conditioner. With a controlled influx and acontrolled drain, it can be ensured that the crank chamber alwayscontains an adequate amount of lubricant.

The swash plate can be constructed so as to be tiltable and axiallydisplaceable in various ways, as seems appropriate to a person skilledin the art. For example, the swash plate can be supported on a Z shaftwith a tilted bore of bearing, and a stroke movement can be superimposedby a rotational movement of the bearing disk, and so on. In oneembodiment of the invention it is proposed that the swash plate besupported on a joint head that can be axially displaced by means of asetting piston incorporated in the adjustment unit, and that the swashplate also be connected by way of an off-centre joint to a componentthat is fixed in the axial direction. A structurally simpler and moreeconomical displacement mechanism can thus be achieved, in which thetilt angle and axial position of the swash plate are related to oneanother in a specified manner. The top-dead-centre point of the pistonwithin the cylinder can be maintained and it becomes possible to avoid adamage space and energy losses, as a result of which the axial pistondrive can be used particularly advantageously as a condenser in an airconditioner. The condenser can be designed as a pure swash-platecondenser or as a wobble-plate condenser. Furthermore, the solution inaccordance with the invention can be employed with gear mechanisms andthe like.

It is advantageous to construct the setting piston and the joint head inone piece, which can provide a saving in extra components as well as theeffort of assembly and expense. The adjustment unit can be so disposedthat it either rotates with the drive shaft, partially or completely, oris unrotatably fixed within a housing. Furthermore, the adjustment unitcan act on the swash plate either from the side away from the piston orfrom the side facing the piston.

DRAWING

Additional advantages will be apparent from the following description ofdrawings that show exemplary embodiments of the invention. The drawings,the description and the claims contain numerous characteristics incombination. A person skilled in the art will be able also to considerthe characteristics individually and to assemble them into other usefulcombinations.

The individual figures show the following:

FIG. 1 an axial piston drive with the piston at the maximal end of itsstroke, in section,

FIG. 2 an axial piston drive with the piston at the minimal end of itsstroke, in section,

FIG. 3 part of a variant according to FIG. 1, and

FIG. 4 a schematic diagram of a form of hydraulic control.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIG. 1 shows an axial piston drive for an air conditioner of a motorvehicle, which operates as a condenser. The axial piston drive comprisesa drive shaft 10 on which a swash plate 16 is supported within a crankchamber 14. The driving action of the swash plate 16 is exerted by wayof hemispherical hinge yokes 56, which connect it to pistons 26, 28 thatare guided within cylinders 22, 24. To absorb tractive and pressureloads, each piston 26, 28 is connected to the swash plate 16 by way oftwo hinge yokes 56, one of which contacts the bearing face 58 that facesaway from the pistons 26, 28 while the other contacts the bearing Face60 that faces towards the pistons 26, 28. The hinge yokes 56 run, by wayof their flat surfaces, along the bearing faces 58, 60 of the swashplate 16 at the full circumferential velocity with superimposed radialmovement, as a result of which an elliptical track is produced. Therounded surfaces of the hinge yokes 56 are seated in sphere shapedformed bearings 62 of the pistons 26, 28, within which there iscomparatively little relative movement during operation.

The swash plate is connected to the drive shaft 10 in a unrotatablyfixed manner, by way of a joint head 48 of a sleeve 64. So that thepiston stroke and hence the ouput of the axial piston drive can becontinuously adjusted, the swash plate 16 is made so that it can betilted on the joint head 48 by means of a controller 18 and moved in theaxial direction along with the sleeve 64. When the tilt angle is large,a long piston stroke and a high output are achieved, while with a smalltilt angle the piston stroke is short and the output low (FIGS. 1 and2).

In accordance with the invention the controller 18 comprises a hydraulicadjustment unit 30 that is separate from the pistons 26, 28. Theadjustment unit 30 incorporates a setting piston 44 formed in one piecewith the sleeve 64 and the joint head 48. The setting piston 44 isguided within a cylinder formed by an adjustor housing 54. The adjustorhousing 54 is attached to the drive shaft 10 in a form-fitting manner,in the radial direction by way of a fitting means not shown here, andaxially by way of a tension ring 76. The drive shaft 10 is axiallysupported in the direction away from the cylinders 22, 24 by theadjustor housing 54, an axial bearing 80 and a running plate 82 set intoa cover 78; in the direction towards the cylinders 22, 24 it restsagainst a housing 86 of the axial piston drive by way of an axial slidebearing 84. The drive shaft 10 is additionally supported in the cover 78and in the housing 86 by way of two radial bearings 88, 90.

The setting piston 44, together with the cylinder, encloses a pressurespace 74 that is sealed off by three seals 68, 70, 72. The swash plate16 is connected to the adjustor housing 54 by way of a joining element66, which is formed integrally with the swash plate 16, and by anoff-centre joint 52.

When compressed oil enters the pressure space 74, the setting piston 44is displaced, together with the sleeve 64, the joint head 48 and theswash plate 16, in the direction towards the cylinders 22, 24, against aprestressed pressure spring 92 (FIG. 2). The pressure spring 92 isnonrotatably attached to the drive shaft 10 and is braced against atension ring 94 in the direction away from the setting piston 44. By theoff-centre joint 52, formed by a bolt 98 that is fixed to the joiningelement 66 and is guided within a slot 96, the stroke movement of theswash plate 16 causes a moment of tilt acting on the swash plate 16.Upon the stroke movement of the swash plate 16 is superimposed a tiltingmovement, guided by the bolt 98 in the slot 96, so that in all cases atop-dead-centre point 100 of the piston 26, 28 within the cylinders 22,24 is preserved. So that only a small amount of oil is required, thevolume of the pressure space 74 is preferably small.

The adjustment unit 30, specifically the setting piston 44, is suppliedwith compressed oil from an oil separator disposed downstream of thecylinders 22, 24, by way of an axial bore 102, 104, 106 in the housing86, in the slide bearing 84, and in the drive shaft 10, and by way of aradial bore 108 in the drive shaft 10 (FIGS. 1, 2 and 4). The compressedoil is advantageously fed into the drive shaft 10 axially, in themiddle. In this region the relative movement between the drive shaft 10and the slide bearing 84 is advantageously made small. Furthermore, theslide bearing 84 can additionally be used as a seal. If no oil pressurehas yet developed in the oil separator 34 when the mechanism is firststarted, the pressure spring 92 sets the tilt angle to maximal, whichensures a build-up of pressure.

The adjustment unit 30 is connected to the oil separator 34 by a influx38 and to the crank chamber 14 by a drain 36. The influx 38 and drain 36can each be controlled by a valve 110, 112. If a higher controllingtorgue is needed, the valve 110 opens. The oil flows at a higherpressure level into the adjustment unit 30 and acts on the settingpiston 44. The valve 112 remains closed during this process. If asmaller controlling torgue is needed, the valve 112 opens, allowing theoil to flow out of the adjustment unit 30 and making less forceavailable in the adjustment unit 30. The swash plate 16 is displaced bythe pressure spring 92 in the direction towards the maximal tilt angle.The valve 110 is closed.

If one of the valves 110, 112 is replaced by a throttle, so that onlythe influx 38 or the drain 36 can be controlled, there canadvantageously be provided an oil-level controller 40 and a channel 42from the oil separator 34 to the crank space 14, as indicated in FIG. 4,to ensure that an adequate amount of lubricant is always available inthe crank space 14.

FIG. 3 shows part of a variant of an axial piston drive with acontroller 20. Components that are substantially the same are in generalidentified by the same reference numerals. Regarding the function andcomponents not shown here, reference is made to the exemplary embodimentin FIGS. 1 and 2. The controller 20 comprises an adjustment unit 32 witha setting piston 46 that is nonrotatably disposed in an annular recess122 in a housing 114 of the axial piston drive. This arrangement makesan additional adjustor housing unnecessary. The setting piston 46 isloaded in the direction towards the swash plate 16 by a firstcompression spring 136, is sealed off from the housing 114 by two seals116, 118 and acts on the swash plate 16 in the axial direction, by wayof a sleeve 120 and a joint head 50 formed integrally with the sleeve120, against a second prestressed, stronger pressure spring 124. In thedirection away from the setting piston 46 the spring 124 is bracedagainst a shoulder 126 of a drive shaft 12. The swash plate 16 is bracedin the axial direction by an off-centre joint (not shown here), so thatthe stroke movement of the swash plate 16 exerts a moment of tilt on theswash plate 16. The setting piston 46 and the sleeve 120 are connectedto one another by an axial bearing 128 that acts on both sides, suchthat the setting piston 46 forms inner bearing faces whereas the sleeve120 and a fastening element 130 form outer bearing faces. With thefastening element 130, which is connected to the sleeve 120 by a screwthread 132, the degree of axial play in the axial bearing 128 can be setto a specified value. The adjustment unit 32, i.e. the setting piston46, is supplied with compressed oil from an oil separator 34 by way ofan axial bore 134, as is the adjustment unit 30 (cf. relevant part ofFIG. 4).

LIST OF REFERENCE NUMERALS

-   10 Drive shaft-   12 Drive shaft-   14 Crank space-   16 Swash plate-   18 Controller-   20 Controller-   22 Cylinder-   24 Cylinder-   26 Piston-   28 Piston-   30 Adjustment unit-   32 Adjustment unit-   34 Oil spearator-   36 Drain-   38 Influx-   40 Oil-level controller-   42 Channel-   44 Setting piston-   46 Setting piston-   48 Joint head-   50 Joint head-   52 Joint-   54 Component-   56 Hinge yoke-   58 Bearing face-   60 Bearing face-   62 Bearing-   64 Joint sleeve-   66 Joining element-   68 Seal-   70 Seal-   72 Seal-   74 Pressure space-   76 Tension ring-   78 Cover-   80 Axial bearing-   82 Thrust washers-   84 Slide bearing-   86 Housing-   88 Bearing-   90 Bearing-   92 Pressure spring-   94 Tension ring-   96 Slot-   98 Bolt-   100 Top-dead-centre-   102 Bore-   104 Bore-   106 Bore-   108 Bore-   110 Valve-   112 Valve-   114 Housing-   116 Seal-   118 Seal-   120 Joint sleeve-   122 Recess-   124 Pressure spring-   126 Shoulder-   128 Bearing-   130 Fastening element-   132 Screw thread-   134 Bore-   136 Pressure spring

1. Axial piston drive with a continuously adjustable piston stroke,comprising a drive shaft (10, 12) on which a swash plate (16) issupported in a crank chamber (14) in such a way as to be tiltable anddisplaceable in the axial direction, and with a controller (18, 20) bymeans of which an tilt angle and an axial position of the swash plate(16) can be adjusted, and with at least one piston (26, 28) connected tothe swash plate (16) so that it can be actuated to move within thecylinder (22, 24), wherein the controller (18, 20) incorporates anadjustment unit (30, 32) that is separated from the piston (26, 28) andhydraulically driven, characterized in that the adjustment unit (30, 32)is supplied with compressed oil by a hydraulic unit which is controlledindependently of the medium being propelled by the piston (26, 28), thehydraulic unit comprises an oil separator (34) disposed downstream ofthe cylinder (22,24).
 2. Axial piston drive according to claim 1,characterized in that the hydraulic adjustment unit (30, 32) isconnected to the crank chamber (14) by way of a drain (36), and a influx(38) from the oil separator (34) to the adjustment unit (30, 32) or thedrain (36) from the adjustment unit (30, 32) to the crank chamber (14)can be controlled.
 3. Axial piston drive according to claim 1,characterized in that in the oil separator (34) and/or in the crankchamber (14) at least part of an oil-level controller (40) is disposedwhich, when a specified oil level in the oil separator (34) is exceededand/or when the oil in the crank chamber (14) falls below a certainlevel, connects the oil separator (34) to the crank chamber (14) by wayof a channel (42).
 4. Axial piston drive according to claim 1,characterized in that in the oil separator and an amount of oil that ispresent are matched to one another in such a way that before an oildeficiency appears in the crank chamber (14), the oil separatoroverflows and the overflowing oil flows back into the crank chamber(14).
 5. Axial piston drive according to claim 1, characterized in thatin the adjustment unit (30, 32) is connected to the crank chamber (14)by way of a drain (36), and a influx (38) from the oil separator (34) tothe adjustment unit (30, 32) and the drain (36) from the adjustment unit(30, 32) to the crank chamber (14) can be controlled.
 6. Axial pistondrive according to claim 1, characterized in that the swash plate (16)is supported on a joint head (48, 50) that can be axially displaced bymeans of a setting piston (44, 46) of the hydraulic adjustment unit (30,32) and the swash plate (16) is connected by an off-centre joint (52) toa component (54) that is fixed in the axial direction.
 7. Axial pistondrive according to claim 6, characterized in that in the setting piston(44) and the joint head (48) are constructed in one piece.
 8. Axialpiston drive with a continuously adjustable piston stroke, comprising adrive shaft (10, 12) on which a swash plate (16) is supported in a crankchamber (14) in such a way as to be tiltable and displaceable in theaxial direction, and with a controller (18, 20) by means of which antilt angle and an axial position of the swash plate (16) can beadjusted, and with at least one piston (26, 28) connected to the swashplate (16) so that it can be actuated to move within a cylinder (22,24), wherein the controller (18, 20) incorporates an adjustment unit(30, 32) that is separated from the piston (26, 28) and hydraulicallydriven, characterized in that the adjustment unit (30, 32) is suppliedwith compressed oil by an oil separator (34) disposed downstream of thecylinder (22, 24) and is connected to the crank chamber (14) by way of adrain (36), and a influx (38) from the oil separator (34) to theadjustment unit (30, 32) or the drain (36) from the adjustment unit (30,32) to the crank chamber (14) can be controlled.
 9. Axial pistonaccording to claim 8 characterized in that in the oil separator (34)and/or in the crank chamber (14) at least part of an oil-levelcontroller (40) is disposed which, when a specified oil level in the oilseparator (34) is exceeded and/or when the oil in the crank chamber (14)falls below a certain level, connects the oil separator (34) to thecrank chamber (14) by way of a channel (42).
 10. Axial piston accordingto claim 8 characterized in that in the oil separator and an amount ofoil that is present are matched to one another in such a way that beforean oil deficiency appears in the crank chamber (14), the oil separatoroverflows and the overflowing oil flows back into the crank chamber(14).
 11. Axial piston according to claim 8 characterized in that in theadjustment unit (30, 32) is connected to the crank chamber (14) by wayof a drain (36), and a influx (38) from the oil separator (34) to theadjustment unit (30, 32) and the drain (36) from the adjustment unit(30, 32) to the crank chamber (14) can be controlled.